We report on specific heat measurements on clean overdoped BaFe 2(As 1-xP x) 2 single crystals performed with a high resolution membrane-based nanocalorimeter. A nonzero residual electronic specific heat coefficient at zero temperature γ r = C/T | T→ 0 is seen for all doping compositions, indicating a considerable fraction of the Fermi surface ungapped or having very deep minima. The remaining superconducting electronic specific heat is analyzed through a two-band s-wave alpha model in order to investigate the gap structure. Close to optimal doping we detect a single zero-temperature gap of Δ 0 ~ 5.3 meV, corresponding to Δmore »0/k BT c ~ 2.2. Increasing the phosphorus concentration x, the main gap reduces till a value of Δ 0 ~ 1.9 meV for x = 0.55 and a second weaker gap becomes evident. From the magnetic field effect on γ r, all samples however show similar behavior γ r(H) $-$ γ r (H = 0) ∝ to H n, with n between 0.6 and 0.7]. This indicates that, despite a considerable redistribution of the gap weights, the total degree of gap anisotropy does not change drastically with doping.« less

We present neutron dffraction analysis of BaFe 2(As 1-xP x) 2 over a wide temperature (10 to 300 K) and compositional (0.11 < x < 0.79) range, including the normal state, the magnetically ordered state, and the superconducting state. The paramagnetic to spin-density wave and orthorhombic to tetragonal transitions are first order and coincident within the sensitivity of our measurements (~ 0:5 K). Extrapolation of the orthorhombic order parameter down to zero suggests that structural quantum criticality cannot exist at compositions higher than x = 0.28, which is much lower than values determined using other methods, but in good agreementmore » with our observations of the actual phase stability range. Lastly, the onset of spin-density wave order shows a stronger structural anomaly than the charge-doped system in the form of an enhancement of the c/a ratio below the transition.« less

We study the antiferromagnetic (AFM) and structural phase transitions in single-crystal BaFe 2(As 1-x P x) 2 (x = 0, 0.3) at temperatures T N and T S, respectively, by high-resolution ac microcalorimetry and SQUID magnetometry. The specific heat measurements of both as-grown and annealed BaFe 2As 2 display a sharp peak at the AFMs-tructural transitions. A kink in the entropy of annealed BaFe 2As 2 gives evidence for splitting of the two transitions by approximately 0.5 K. No additional features could be identified in the specific heat of both BaFe 2As 2 and BaFe 2(As 0.7P 0.3) 2 inmore » the temperature regions around T* > T S where torque measurements [S. Kasahara et al., Nature 486, 382 (2012)] had revealed the "true" nematic phase transition, even though the Ginzburg-Landau model used to fit the magnetic torque data indicates that the expected thermal anomaly should be easily observable with our experimental resolution.« less

In this paper, irradiation with 4 MeV protons was used to systematically introduce defects in single crystals of the iron-arsenide superconductor BaFe 2(As 1-xP x) 2, x = 0.33. The effect of disorder on the low-temperature behavior of the London penetration depth λ(T) and transition temperature T c was investigated. In nearly optimally doped samples with T c ~ 29 K, signatures of a superconducting gap with nodes were observed. Contrary to previous reports on electron-irradiated crystals, we do not see a disorder-driven lifting of accidental nodes, and we observe that proton-induced defects are weaker pair breakers than electron-induced defects.more » Finally, we attribute our findings to anisotropic electron scattering caused by proton irradiation defects.« less

We investigate the electronic specific heat of superoptimally substituted BaFe 2(As 1-x P x) 2 single crystals in the superconducting state using high-resolution nanocalorimetry. From the measurements, we extract the substitution dependence of the condensation energy, superconducting gap Δ, and related microscopic parameters. We find that the anomalous scaling of the specific heat jump ΔC ∝ to Tmore » $$3\atop{c}$$ , found in many iron-based superconductors, in this system originates from a T c-dependent ratio Delta/k BT c in combination with a substitution-dependent density of states N(epsilon(F)). A clear enhancement is seen in the effective mass m* as the composition approaches the value that has been associated with a quantum critical point at optimum substitution. However, a simultaneous increase in the superconducting carrier concentration n s yields a penetration depth lambda that decreases with increasing T c without sharp divergence at the quantum critical point. Uemura scaling indicates that T c is governed by the Fermi temperature T F for this multiband system.« less